Nuclear quadrupole double resonance spectroscopy will be applied to study systems of interest in biology and medicine. The technique is applicable to nuclei of spin I greater than of equal to l, e.g., 14N, 17O, 2D, 67Zn. 14N NQR will be employed to study nucleotides, nucleosides and related heterocyclic bases, as well as their metal complexes; to study binding of amino acids and peptides to metal ions, and to study porphyrin-metal ion complexes. Double resonance NQR methods will be employed to study 17O, 14N and 2H NQR spectra in hydrogen bonding systems. The NQR spectroscopy of 17O in natural adundance is a promising technique for the study of the electronic and structural environment about this important element; the method will be applied to a wide range of biochemically interesting substances. The quadrupole double resonance spectra of metallic nuclei, notably 67Zn, in a wide range of complexes will be obtained, as a prelude to study of Zn binding to proteins.